This invention relates to the control of power levels of transmitted signals in telecommunication systems, in particular spread spectrum multiple access systems.
Good transmit power control methods can be important to communication systems having many simultaneous transmitters because such methods reduce the mutual interference of such transmitters. For example, transmit power control is necessary to obtain high system capacity in communication systems that use code division multiple access (CDMA). This is important for the uplink, i.e., for transmissions from a remote terminal to the network, e.g., a base station. Uplinks are also sometimes called reverse links.
The need for transmit power control in the uplink is recognized in current CDMA cellular systems. Uplink power control according to the IS-95-A standard is provided by a closed-loop method, in which a base station measures the strength of a signal received from a remote station and then transmits one power control bit to the remote station every 1.25 milliseconds. Based on the power control bit, the remote station increases or decreases its transmit (uplink) power by a predetermined amount.
The need for transmit power control for the downlink, i.e., for transmission from the network to a remote station, has been deemed less important in current cellular and other CDMA communication systems.
A trivial form of downlink power control would be provided by a communication system in which a remote terminal measures its received downlink power level and simply reports the measurement to a base station, which might adjust its transmit power in predetermined circumstances. Such a communication system is among those described in International Patent Publication No. WO 95/12297 by Gilhousen et al., which also describes a communication system in which downlink transmit power level is reduced by a predetermined amount based on frame error rate measurements, received uplink power levels, or received downlink power levels.
Downlink power control in a system compliant with the IS-95-A standard is based on frame error rate (FER) measurements by the remote station, which sends FER reports to the system. Sections 6.6.4.1.1 and 7.6.4.1.1 of the IS-95-A standard note that such FER reports can be sent when a threshold has been crossed and/or periodically. (Typically, an FER report would be sent every 1-5 seconds.) One problem with this method is that it can take a long time (several seconds) to accumulate the appropriate FER statistics. As a result, it is impossible to track Rayleigh fading and shadow fading. The method has proved to be so slow that it is usually attributed hardly any gain compared to not using downlink power control.
Some newer personal communications systems (PCS) also use CDMA. The features of U.S. PCS systems are specified in "Personal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communications Systems", ANSI J-STD-008 (August 1995), which is similar to the cellular IS-95 standard in many respects. For operation with rate set 2, however, the J-STD-008 standard requires the remote station to report downlink frame errors whenever they occur. This puts the network in complete control of frame errors, but it still takes a long time to accumulate the appropriate statistics, yielding only slight improvement over the IS-95-A standard's method.
In other communication system concepts like CODIT, which is described in "Final Report on Radio Subsystem Functionality", R2020/CSE/LC/DS/P/047/al, UMTS Code Division Testbed (CODIT), CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. ed. (August 1995), the signal quality is determined by estimating the raw bit error rate (BER) instead of the FER. Hence, good statistics can be obtained faster, and a remote station sends BER reports to the network more often (typically, 1-10 times per second). System performance is considerably improved in comparison to a system using downlink transmit power control according to the IS-95-A standard, but the CODIT method is still too slow to handle Rayleigh fading.
One might use the uplink transmit power control method described in the IS-95-A standard for transmit power control in the downlink. This is described in European Patent Publication No. 0 680 160 by Dohi et al. The remote terminal would then measure the downlink signal to interference ratio (SIR) and transmit an appropriate power control command on the uplink. In accordance with the IS-95-A standard, each power control command would be a single bit that was uncoded in order to minimize signalling overhead. Nevertheless, the communication system to which European No. 0 680 160 is directed has some significant differences from the system specified by the IS-95-A standard. For example, the European system has a frame length that is half that of IS-95-A, a bit rate of several hundred kilobits per second, a wider channel bandwidth of 5 MHZ, and a CDMA chip rate of four million chips per second.
Such a communication system would track Rayleigh fading fairly well, and might work well when the remote terminal is not in a soft handoff mode, i.e., when the remote terminal is not communicating with two or more base stations simultaneously. This is sometimes also referred to as macrodiversity. Soft handoff is described in U.S. Pat. No. 5,109,528 to Uddenfeldt and U.S. Pat. No. 5,327,577 to Uddenfeldt, both of which are expressly incorporated here by reference. When the remote terminal is not in soft handoff mode, the error rate of the uncoded power control commands would typically be about one percent, which would not cause any great problems.
Nevertheless, the error rate of the uncoded downlink power control commands can be expected to increase significantly when the remote terminal is in soft handoff mode. In addition, the errors in the commands received in different base stations involved in the soft handoff will be almost independent. Since 1600 power control commands per second would be sent to two base stations involved in a soft handoff according to the system described by Dohi et al., the commanded transmit power levels of the base stations can be expected to drift with respect to each other to levels that may be suboptimal from a system-capacity point of view. The lost capacity occurs because at least one of the base stations in communication with a remote station in soft handoff mode will transmit at a power level that is too high.
One way to solve the problems encountered in downlink transmit power control is by making frequent adjustments of the downlink power levels transmitted from the base stations involved in a soft handoff. These base stations send the latest values of their downlink transmit power levels to a radio network controller (RNC) which those values and sends respective adjustment commands .DELTA.P.sub.1.sup.DL (t.sub.n) and .DELTA.P.sub.2.sup.DL (t.sub.n) back to the base stations. This solution is disclosed in U.S. patent application No. 08/870,867, filed Jun. 6, 1996, and herein incorporated by reference.
This method is illustrated in FIG. 1, in which a base station BS1 sends a message P.sub.1.sup.DL (t.sub.n), indicating its downlink transmit power level P.sub.1 (t.sub.n) at time t.sub.n, to a radio network controller (RNC). In a cellular communication system, the RNC might be a base station controller or a mobile services switching center. Also, a base station BS2 sends a message P.sub.2.sup.DL (t.sub.n), indicating its downlink transmit power level P.sub.2 (t.sub.n) at time t.sub.n, to the RNC. In the soft handoff mode shown in FIG. 1, a remote station RT receives signals from the base station BS1 and receives signals having substantially the same message information from the base station BS2.
The RNC sends respective adjustment commands .DELTA.P.sub.1.sup.DL (t.sub.n) and .DELTA.P.sub.2.sup.DL (t.sub.n) to the base stations that are based on the messages P.sub.1.sup.DL (t.sub.n), P.sub.2.sup.DL (t.sub.n), causing the base stations to adjust their downlink transmit power levels so that a desired relation between P.sub.1 (t.sub.n+1) and P.sub.2 (t.sub.n+1)is maintained. The transmit power levels P.sub.1 (t.sub.n), P.sub.2 (t.sub.n) result from earlier commands .DELTA.P.sub.1.sup.DL (t.sub.n-1) and P.sub.2.sup.DL (t.sub.n-1), as shown in FIG. 1.
The RNC can determine the adjustment commands in several ways, as described in U.S. patent application Ser. No. 08/870,867. The power level adjustments typically are made once per frame, keeping the drift between the downlink transmit power levels of the downlink transmitters involved in a soft handoff to a low level, e.g., less than 1 dB on average.
How the independent errors in a remote station's power control commands come about is illustrated in more detail in FIG. 2, which shows a remote terminal RT in communication with two base stations BS1, BS2 that are under the control of an RNC. In the soft handoff mode shown in FIG. 2, as in FIG. 1, the remote station RT receives signals from the base station BS1 with a power level P.sub.1 and receives signals having substantially the same message information from the base station BS2 with a power level P.sub.2. Power control commands PC transmitted by the remote station RT are received by the base station BS1 as commands PC' and are received by the base station BS2 as commands PC". Due to errors that may arise from many sources, a command may not be the same as either or both of the commands PC', PC", and the command PC' may not be the same as the command PC".
The magnitude of the difference between one downlink power level and another downlink power level affects the system's capacity because one base station's transmissions look like interference to the other base station. In addition, the rate that each downlink power level drifts is important because faster rates usually require the control commands to be issued more frequently. This increases the messaging load that must be carried by the links between the base stations BS1, BS2 and the RNC.
According to U.S. patent application Ser. No. 08/870,867, these problems are avoided by using quality-based downlink power control. According to this approach, the remote terminal RT measures the quality of the received downlink signal, either by determining the FER or the BER, and then the remote terminal RT reports its quality measurement to the network by sending a suitable quality message to the base stations involved in the handoff. Each base station forwards the remote terminal's quality message (a Layer-3 message) to the RNC, which causes the base stations to adjust their downlink transmit powers appropriately.
According to U.S. patent application Ser. No. 08/870,867, the remote station RT either may periodically send reports of its received downlink signal strength to the network or may send a report whenever a signal from a new downlink transmitter is received with a strength that is higher than any other currently received downlink signal strength. In such a communication system, the network RNC would respond to the remote station's signal strength reports by causing only the base station whose downlink signal is received with the highest strength at the remote terminal RT to transmit.
This scenario is depicted in FIG. 3, which shows that when the remote terminal's received downlink power level P.sub.1 from the base station BS1 is greater than the received downlink power level P.sub.2 from the base station BS2, the downlink power level P.sub.1 transmitted by the base station BS1 is greater than zero, and the downlink power level P.sub.2 transmitted by the base station BS2 is zero. (In practice, the power level P.sub.2 only needs to be substantially less than the power level P.sub.1.) The weaker base station BS2 should operate otherwise normally in soft handoff mode. The network informs the remote terminal RT whenever the system decides to change the transmitting base station to accommodate base station differences, such as different pseudonoise signals, although different base stations generally look the same from the point of view of the remote station in soft handoff mode.
Yet another solution disclosed in U.S. patent application Ser. No. 08/870,867 is to apply open loop power control instead of closed loop power control during the macrodiversity connection. According to this approach, the uplink SIR is measured for each frame in the base stations involved in a soft handoff, and the SIR measurements are forwarded to the RNC. The RNC processes the SIR values from the base stations involved in the soft handoff and causes only that base station to transmit whose downlink power level received at the remote terminal is highest.
This scenario is depicted in FIG. 4, which shows that the downlink power level P.sub.1 (t.sub.n+1) transmitted by the base station BS1 at a time t.sub.n+1 is greater than zero, and the downlink power level P.sub.2 (t.sub.n+1) transmitted by the base station BS2 at the time t.sub.n+1 is zero. This condition occurs as a result of respective downlink transmit power control set commands SetP.sub.1.sup.DL, SetP.sub.2.sup.DL sent by the RNC to the base stations BS1, BS2, respectively. The RNC generates the power set commands based on the base stations' respective determinations and reports of the uplink SIR values SIR.sub.1 (t.sub.n), SIR.sub.2 (t.sub.n) at the preceding time t.sub.n.
Hence, on occasions when the uplink power received at a base station, e.g., station BS1, is higher than the uplink power received at another base station, e.g., station BS2, the RNC sends messages to the base station BS1 causing the station BS1 to start transmitting to the remote terminal RT, to the base station BS2 causing the station BS2 to stop transmitting to the remote terminal RT, and to the remote terminal RT causing the terminal RT to listen only to base station BS1 and not to base station BS2 from a specific frame number onwards.
FIGS. 5A, 5B, and 5C further illustrate operation of a communication system in accordance with U.S. patent application Ser. No. 08/870,867. In block 501, base station BS1 receives data and transmit power adjustment commands from a mobile station RT. The base station BS1 identifies and decodes an adjustment command received at a first time instant, which conveniently may have values of either+1 or-1, and accordingly adjusts its downlink transmit power level at the next time instant. As indicated by block 503 in FIG. 5A, for example, if the mobile sends adjustment command PC=1, then the base station's transmit power level P.sub.1 (t.sub.n+1)=+1 dB with respect to its previous transmit power level P.sub.1 (t.sub.n). Similarly, if the mobile sends adjustment command PC=-1, then the base station's transmit power level P.sub.1 (t.sub.n+1)=-1 dB with respect to its previous transmit power level P.sub.1 (t.sub.n). In block 505, the message P.sub.1.sup.DL (t.sub.n) indicating the downlink transmit power level P.sub.1 (t.sub.n) at time t.sub.n is sent by the base station BS1 to the RNC. The base station preferably sends such messages not as often as the mobile sends power adjustment commands in order to reduce the signalling load on the base station--controller connection. For example, the base station may count the number of received adjustment commands n and send a message every N-th adjustment command, where N is a factor a times the number n. The factor a may be an integer in the range from about 16 to about 1600 for some communication systems. In block 507, the base station BS1 sets its downlink transmit power level to the commanded amount.
As indicated by block 509 in FIG. 5B, the RNC receives the messages P.sub.1.sup.DL (t.sub.n) and possibly messages from other base stations involved in a soft handoff, e.g., P.sub.2.sup.DL (t.sub.n). The RNC, which may include specialized circuitry or a programmable processor for the purpose, then determines new respective adjustment commands .DELTA.P.sub.1.sup.DL (t.sub.n) and .DELTA.P.sub.2.sup.DL (t.sub.n+1), which as indicated by block 513 in FIG. 5B may be based on the difference (block 511) between the downlink transmit power levels P.sub.1 (t.sub.n), P.sub.2 (t.sub.n) of the base stations BS1, BS2. In the example illustrated, P.sub.1.sup.DL (t.sub.n)&gt;P.sub.2.sup.DL (t.sub.n), and thus the new adjustment command .DELTA.P.sub.1.sup.DL (t.sub.n+1) corresponds to the first base station's previous power level less half of the difference between the power levels, and the new adjustment command .DELTA.P.sub.2.sup.DL (t.sub.n+1) corresponds to the second base station's previous power level plus half of the same difference. If P.sub.2.sup.DL (t.sub.n)&gt;P.sub.1.sup.DL (t.sub.n), substantially the same operation would be carried out, but the identities of the base stations would be interchanged. These commands are then communicated in any convenient way to the respective base stations as indicated by block 515.
The operation of this kind of communication system is further illustrated by FIG. 5C, in which an adjustment command .DELTA.P.sub.1.sup.DL from the RNC is received by the base station BS1 as indicated by block 517. In response, the base station adjusts its transmit power level at a time t.sub.n +t.sub.m as indicated by block 519, where the interval t.sub.m is a convenient time interval, such as an integer number of communication frames. Similar operation are carried out in the other base station (station BS2). In addition, if the power level of the weaker base station was commanded to be substantially zero by the operations indicated by blocks 511-519, the weaker base station might advantageously be maintained in soft handoff mode in all other respects as described above.
The closed loop downlink power control method described above with reference to FIG. 1 involves extra signalling between the base stations and the RNC. For example, the base stations need to send a power message to the RNC about every 10 msec in order to keep drift at around 1-2 dB in a system with a high rate uncoded or little coded power control commands and a power adjustment step size of 1 dB. While the other solutions disclosed with reference to FIGS. 3 and 4 result in a fairly low amount of control signalling between the RNC and the base stations, there remains a need for efficient closed loop power control combined with a method of sending power control data from an RNC to associated base stations during macrodiversity transmissions.